THE 26.2-MILE Boston Marathon course from Hopkinton to Copley Square and the journey from trauma to recovery are running on parallel tracks this year. On Monday, runners will face challenges over the period of a few hours that reflect the difficulties confronted over the last 12 months by spectators who suffered direct and indirect trauma after terrorists detonated two bombs at last year’s finish line, killing three, maiming scores, and inflicting psychological harm on a city during its premier, communal event.

Emotionally, the Marathon has always been unidirectional. Spectators, especially those closest to the finish line, could always empathize with the pain and duress of the runners. Runners drew what energy they could from the crowd. It’s fair to say they used the crowd.

This year the relationship has changed. How serious, after all, are runners’ foot blisters and leg cramps if measured against spectators who lost their feet and legs in last year’s bombings? It’s a new Marathon covenant. Awe and empathy now must flow in both directions.

Marathon runners often report spending a restless or even sleepless night before the race. They’re a bundle of nerves at the starting line, kept physically in check only by the mass of other runners pressing in on them. Barely underway, runners quickly confront a drop of 130 feet, the steepest downhill along the race course. Precious little thinking takes place at this point, according to Rusconi. Runners are in a haze.

McGrath describes the first stage of the traumatic experience as “circuit breaking.’’ Brain activity reduces as the victim’s system shuts down to compensate for all of the danger, pain, and craziness around them. Many people noticed that some of the most seriously injured spectators at last year’s Marathon displayed an oddly serene countenance. They had entered what McGrath calls the “trauma zone.’’

The pack begins to thin out over the next eight miles through Hopkinton, Ashland, and Framingham. The terrain is mostly forgiving, with gentle dips and gradual ascents with only a few minor perils to negotiate, such as train tracks. Still, the mainly flat surface gives some runners a false sense of security. At miles 9 through 12, placid Lake Cochituate and neat New England-style homes come into view. Natick spectators provide plenty of support along the way. Rusconi cautions runners about getting too far ahead of their pace during this stage of the race. They can damage themselves.

Marathon runners often report spending a restless night before the race. In the first stage of a traumatic experience, Dr. Ellen McGrath says brain activity reduces as the victim’s system shuts down to compensate for all of the danger, pain, and craziness around them.

McGrath identifies the second stage of trauma recovery as “the return of feelings.’’ Some victims want to return to the site of the traumatic event during this period. And nearly all victims benefit from talking in minute detail about their experiences. For some people, the emotions trickle out. For others, they ebb and flow. But for those who repress their feelings, huge waves of emotion can arrive without warning, overwhelming the victim.

The crowd noise picks up during miles 12 through 15 in Wellesley. The terrain and the people are friendly, with lots of reminders that the runners are halfway home. Runners also need to remind themselves that they are closing in on the most difficult part of the race — 9 miles of tough ups and downs starting with a 100-foot descent to Newton Lower Falls. Less-experienced marathoners can crack at miles 16 and 17 in Newton, which includes a half-mile long hill and a barren overpass over Route 128 where runners are exposed to crosswinds. Rusconi notes that even many veteran marathoners experience miles 16 and 17 as tougher than Heartbreak Hill at mile 20.

Dina Rudick/Globe Staff/File 2008

Ben Reynolds, of Chatham, N.J., raced through Kenmore Square.

McGrath describes the third stage in trauma recovery as “constructive action’’ when survivors take back control and start to overcome their sense of powerlessness. Victims want to meet their rescuers and connect with other trauma sufferers and their families. Often they go back to work. Such actions, writes McGrath, become “an engine that propels you forward.’’

The next few miles through Boston College and Brookline are largely downhill, but nothing comes easily after 23 miles of pavement pounding. A small grade at mile 25 can appear insurmountable. But then, at the crest, Fenway Park comes into view. Huge, roaring crowds await in Kenmore Square. Signs indicate just 1 mile to go. Soon comes the vaunted right turn onto Hereford Street, which Rusconi depicts as a “scream tunnel’’ in the joyful sense of the term. Finally the left turn at Boylston Street and the blue and yellow finish line.

The last piece in trauma recovery is what McGrath calls “reintegration.” People are smarter, stronger, and more insightful for the experience. A lash of new bone has formed around “the broken bones of the soul.’’

This year’s Marathon stands for regeneration, a concept that might be difficult to upload in photos and videos along the race route. But the message will be communicated over every step of the race. Runners and spectators are able to move forward together because the bombings and their aftermath — in the deepest sense — fused them into a state of wholeness.

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